1. Field of the Invention
The present invention relates to methods and apparatus for wellbore completions. More particularly, the invention relates to completing a wellbore by expanding tubulars therein. More particularly still, the invention relates to completing a wellbore by separating an upper portion of a tubular from a lower portion after the lower portion of the tubular has been expanded into frictional engagement with another tubular there around.
2. Description of the Related Art
Hydrocarbon and other wells are completed by forming a borehole in the earth and then lining the borehole with steel pipe or casing to form a wellbore. After a section of wellbore is formed by drilling, a section of casing is lowered into the wellbore and temporarily hung therein from the surface of the well. Using apparatus known in the art, the casing is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. In this respect, the first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or xe2x80x9chung off of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever decreasing diameter.
Apparatus and methods are emerging that permit tubulars to be expanded in situ. The apparatus typically includes expander tools which are fluid powered and are run into a wellbore on a working string. The hydraulic expander tools include radially expandable members which, through fluid pressure, are urged outward radially from the body of the expander tool and into contact with a tubular therearound. As sufficient pressure is generated on a piston surface behind these expansion members, the tubular being acted upon by the expansion tool is expanded past its point of plastic deformation. In this manner, the inner and outer diameter of the tubular is increased in the wellbore. By rotating the expander tool in the wellbore and/or moving the expander tool axially in the wellbore with the expansion member actuated, a tubular can be expanded along a predetermined length in a wellbore.
There are advantages to expanding a tubular within a wellbore. For example, expanding a first tubular into contact with a second tubular therearound eliminates the need for a conventional slip assembly. With the elimination of the slip assembly, the annular space required to house the slip assembly between the two tubulars can be reduced.
In one example of utilizing an expansion tool and expansion technology, a liner can be hung off of an existing string of casing without the use of a conventional slip assembly. A new section of liner is run into the wellbore using a run-in string. As the assembly reaches that depth in the wellbore where the liner is to be hung, the new liner is cemented in place. Before the cement sets, an expander tool is actuated and the liner is expanded into contact with the existing casing therearound. By rotating the expander tool in place, the new lower string of casing can be fixed onto the previous upper string of casing, and the annular area between the two tubulars is sealed.
There are problems associated with the installation of a second string of casing in a wellbore using an expander tool. Because the weight of the casing must be borne by the run-in string during cementing and expansion, there is necessarily a portion of surplus casing remaining above the expanded portion. In order to properly complete the well, that section of surplus unexpanded casing must be removed in order to provide a clear path through the wellbore in the area of transition between the first and second casing strings.
Known methods for severing a string of casing in a wellbore present various drawbacks. For example, a severing tool may be run into the wellbore that includes cutters which extend into contact with the tubular to be severed. The cutters typically pivot away from a body of the cutter. Thereafter, through rotation the cutters eventually sever the tubular. This approach requires a separate trip into the wellbore, and the severing tool can become binded and otherwise malfunction. The severing tool can also interfere with the upper string of casing. Another approach to severing a tubular in a wellbore includes either explosives or chemicals. These approaches likewise require a separate trip into the wellbore, and involve the expense and inconvenience of transporting and using additional chemicals during well completion. These methods also create a risk of interfering with the upper string of casing. Another possible approach is to use a separate fluid powered tool, like an expansion tool wherein one of the expansion members is equipped with some type of rotary cutter. This approach, however, requires yet another specialized tool and manipulation of the run-in string in the wellbore in order to place the cutting tool adjacent that part of the tubular to be severed. The approach presents the technical problem of operating two expansion tools selectively with a single tubular string.
There is a need, therefore, for an improved apparatus and method for severing an upper portion of a string of casing after the casing has been set in a wellbore by expansion means. There is a further need for an improved method and apparatus for severing a tubular in a wellbore. There is yet a further need for a method and apparatus to quickly and simply sever a tubular in a wellbore without a separate trip into the wellbore and without endangering the integrity of the upper string of casing.
The present invention provides methods and apparatus for completing a wellbore. According to the present invention, an expansion assembly is run into a wellbore on a run-in string. The expansion assembly comprises a lower string of casing to be hung in the wellbore, and an expander tool disposed at an upper end thereof. The expander tool preferably includes a plurality of expansion members which are radially disposed around a body of the tool in a spiraling arrangement. In addition, the lower string of casing includes a scribe placed in the lower string of casing at the point of desired severance. The scribe creates a point of structural weakness within the wall of the casing so that it fails upon expansion.
The expander tool is run into the wellbore to a predetermined depth where the lower string of casing is to be hung. In this respect, a top portion of the lower string of casing, including the scribe, is positioned to overlap a bottom portion of an upper string of casing already set in the wellbore. In this manner, the scribe in the lower string of casing is positioned downhole at the depth where the two strings of casing overlap. Cement is injected through the run-in string and circulated into the annular area between the lower string of casing and the formation. Cement is further circulated into the annulus where the lower and upper strings of casing overlap. Before the cement cures, the expansion members at a lower portion of the expansion tool are actuated so as to expand the lower string of casing into the existing upper string at a point below the scribe. As the uppermost expansion members extend radially outward into contact with the casing, including those at the depth of the scribe, the scribe causes the casing to be severed. Thereafter, with the lower string of casing expanded into frictional and sealing relationship with the existing upper casing string, the expansion tool and run-in string, are pulled from the wellbore.